It has been known to protect surfaces such as steel surfaces with an elemental metal coating such as a zinc electroplate or galvanized zinc coating. Such zinc surfaces can then be treated by traditional chromate coatings. Also, chromium-containing coating compositions which further contain pulverulent zinc and are substantially resin-free are particularly desirable for providing a substrate such as a ferrous substrate with corrosion resistance.
All such coatings find utility for coating small metal parts, e.g., fasteners and the like and are especially useful in the automotive industry. When such parts are offered to an industry such as the automotive industry, wherein the substrate is protected with a coating composite, a great variety of choice can be manifested. It is, for example, known to coat hexavalent-chromium-containing and pulverulent-zinc-containing undercoatings with silicate topcoatings, as disclosed in U.S. Pat. No. 4,365,003. It is also known in the protection of zinc surfaces such as galvanized sheets, which have been first treated by traditional chromate coating, to topcoat the treated surface with potassium or sodium silicate, as has been discussed in Japanese Patent Disclosure No.: Showa 53-125239. Zinc plated articles can be protected by coating with an aqueous solution of potassium silicate containing an organic dye as has been disclosed in Japanese Patent Disclosure No.: Showa 80-030593. Further to the protection of zinc plate, the silicate solutions can comprise aggressive chemical environments, e.g., containing sulfuric acid and hydrogen peroxide, as discussed for example in U.S. Pat. No. 4,222,779, and nevertheless contain dye as disclosed in U.S. Pat. No. 4,225,350.
In an industry such as the automotive industry where parts can be galvanized or zinc plated or bear chromium and zinc containing coatings, it would be desirable for such parts to not only bear a topcoating but, to also have coating uniformity. It would thus be desirable to have such variety of parts exhibit uniform corrosion and heat resistance, for example, as well as other desirable attributes, especially torque control for fasteners.